This invention relates generally to electromagnetic flowmeters, and more particularly to a flowmeter whose electrodes have a relatively large surface area and whose electromagnet coils are excited by a low-frequency voltage.
In the conventional electromagnetic flowmeter, the fluid to be metered is conducted through a flow tube having a pair of diametrically-opposed point electrodes. The fluid intersects an electromagnetic field, thereby inducing an electromotive force in the fluid to generate an electrode signal whose magnitude is a function of flow rate.
In order to minimize the effect of the flow velocity profile on the accuracy of an electromagnetic flowmeter, it is known to use in lieu of point electrodes, arcuate electrodes or electrodes having a button-shape form. The reason for this is that the undesirable effects of a non-uniform flow velocity profile on meter accuracy can be eliminated by using electrodes having a large surface area, in that the electromotive force then induced in the fluid is averaged and a force proportional to average flow velocity is produced. Thus in the case of a flowmeter having a flow tube of rectangular cross-section, the influence of the resultant non-uniform flow velocity profile on meter precision can be minimized by using electrodes of large surface area.
Despite the distinct advantages gained by using electrodes of large surface area, their use has not been widespread and has heretofore been more or less restricted to flow measurement under high-velocity flow conditions. The reason for this lack of popularity is that electrodes having a larger surface area adversely affect the zero point stability of the meter. When, as is generally the case, the electromagnet coils of the flowmeter are excited at the frequency of the commercial power line (i.e., 50 or 60 Hz), this excitation induces eddy currents which flow into the electrodes. These eddy currents become greater with larger electrode surfaces, thereby introducing a significant zero drift error.
To overcome this drawback, it has heretofore been the practice to provide an excitation system for the electromagnet coils which produces the low-frequency wave whose frequency is much lower than the usual 50 or 60 Hz commercial power line frequency. In this known system, a low-frequency wave such as 10 Hz is generated by a low-frequency oscillator energized by a DC power source. The output of the low-frequency oscillator is amplified and then applied to the excitation coils of the electromagnetic flowmeter.
Though with an excitation system of this known type one is able to obviate the problem of eddy currents and thereby provide zero point stability while using larger surface area electrodes to eliminate the undesirable effects encountered with a non-uniform velocity profile, this system suffers from certain practical disabilities.
In the case of an electromagnetic flowmeter having a larger diameter flow tube, the power requirements for a meter of this type are considerable, and this dictates an excitation system that includes a DC source of substantial size for the low-frequency oscillator, a large scale power amplifier, and so on.
And even though a flowmeter of larger diameter usually precludes the employment of an excitation system having a large DC source, the fact remains that such meters must often be installed in difficult situations which give rise to a non-uniform flow velocity profile, such as an installation adjacent an elbow in the flow line which acts to introduce considerable disturbances in the fluid velocity profile. Yet while large diameter meters are rendered inaccurate by the non-uniform flow velocity profile to which they are subjected, it is not feasible to use a known type of excitation system in conjunction with large surface electrodes to eliminate this problem.